1. Field of the Invention
This invention relates to miniature optical display devices, and more particularly to charge coupled device (CCD) liquid crystal light valves (LCLVs).
2. Description of the Related Art
There is a need for miniature vision enhancement devices that is not presently being satisfied. Applications for such a device include use in goggles for the vision impaired, and helmet-mounted miniature displays for night use.
Presently available "night scope" goggles are on the order of three to four inches thick, which is uncomfortably big and deters people from using them. Such goggles require a 180.degree. rotation of the image, performed by optical fibers that add about an inch to the overall goggle thickness.
Existing technologies for image display also have relatively large geometries, typically with apertures extending over several cms, and cannot be easily adapted to the miniaturization required in devices for the vision impaired. One problem of vision impairment involves localized blind spots on the retina. It would be desirable to distort and redirect an incoming image away from the blind spots and onto the functioning areas of the retina, but existing goggles do not have an image processing capability; they merely detect and display images.
A miniature optical display device known as a charge coupled device (CCD) liquid crystal light valve (LCLV) has been developed recently, but it does not have an imaging capability. It is described for example in Welkowsky et. al., "Status of the Hughes Charge-Coupled-Device-Addressed Liquid Crystal Light Valve", Optical Engineering, Vol. 26, No. 5, pages 414-417, May 1987. In this type of device, shown in FIGS. 1 and 2, a CCD integrated circuit 2 is fabricated on one side of a semiconductor wafer 4, generally silicon, and is used to supply a spatially resolved signal to a light valve structure on the other side of the wafer. The CCD circuits convert a serial input voltage signal into sampled charge packets and distribute them onto a regular two-dimensional array, which may typically be 256.times.256 pixels with present configurations. A readout structure 6 transports the charge information from the wafer's epitaxial layer, upon which the CCD is formed, to the opposite side of the wafer while retaining the spatial resolution of the charge packets. A mirror, such as dielectric mirror 8, is provided between the readout structure and a liquid crystal cell 10. A transparent electrode 12, generally of indium tin oxide (ITO), is formed on the other side of the liquid crystal cell and is capped with a glass coverplate 14.
A bias voltage from voltage source 16 is applied across the readout structure and electrode 12. To avoid deterioration of the liquid crystals, an alternating bias is used. The readout structure in effect serves as a spatial voltage divider, causing the bias voltage to be applied across the liquid crystal cell in proportion to the CCD signal for each pixel. A readout light beam 18 is directed through the liquid crystal cell to the mirror, which reflects a high percentage of the readout light while greatly attenuating the non-reflected portion to prevent activation of the silicon substrate, which is photosensitive. The electro-optic liquid crystal converts the amount of charge in each of the CCD packets into a proportional amount of spatial modulation of the readout light.
A cross-section of the light valve which shows the readout structure is given in FIG. 2. This structure consists of a high resistivity silicon substrate 20, a microdiode focusing grid 22, a guard ring diode 24, and an MOS gate oxide layer 26 with the microdiodes 22 on one side and the dielectric mirror 8 on the other side. The CCD gate electrodes 28 are formed on an SiO.sub.2 layer 30, which in turn overlies the p-type epitaxial layer 32. The CCD buckets 34 are defined in the epitaxial layer by the CCD gate electrodes 28 and by CCD epitaxial layer channel stops 36.
The dielectric mirror 8 is composed of multiple alternating pairs of 1/4 wavelength Si and SiO.sub.2 layers, tuned to the wavelength of the incident readout light. The liquid crystal in cell 10, which performs the electro-optic modulation, is generally twisted nematic. The readout light is polarized in a plane of polarization that is rotated in direct proportion to the amount of signal charge in each pixel which activates the liquid crystal. Thus, a spatially resolved pattern of polarization modulation is introduced onto the reflected readout light, which may then be converted into an intensity-modulated output image by the use of a downstream polarizer in a 90.degree. analyzer optical configuration.
The CCD circuits themselves are symbolically represented in FIG. 3. They convert a serial electrical input voltage signal into a two-dimensional parallel array of charge packets using a four-phase clocking design. A serial input signal is entered through an input amplifier 38 into a serial input register 40, in which the charge samples are clocked one cell at a time to the right. When the serial input register 40 is full, clocking of charge stops and each of the charge samples is simultaneously shifted into a serial-to-parallel transfer structure 42. Upon completion of the transfer, the CCD begins clocking a new line of information into the serial register 40. While this new line is being clocked into place, the line of charge packets in the transfer structure 42 is shifted down by one line into the CCD parallel array 44 to accept the next line of charge packet information. This process continues (unless commanded to stop) until the entire parallel array CCD is filled.
When all shifting of charge has been completed and the CCD array is full, each line of charge is held under corresponding lines of gates in the parallel array. The voltage on these gates is slowly reduced to release all of the charge packets, which diffuse through the epitaxial layer to be transported to the opposite side of the silicon chip by the readout structure. If desired, a serial output register 46 and output amplifier 48 may be used to clock out from the CCD lines of charge packets that are not transmitted through the epitaxial layer.
The CCD-LCLV was developed for use as an input device in coherent optical data processing systems. Its serial electrical input allows it to form a precise optical display. However, it is not designed to receive and process an input optical image. The use of a CCD-addressed LCLV as an imager device was demonstrated in 1987, and is described in Efron et. al., "A Submicron Metal Grid Mirror Liquid Crystal Light Valve for Optical Processing Applications", SPIE, Vol. 1151, 1989, pages 591-606, and particularly page 595. In this application, the CCD was used in an inverse mode. The silicon substrate was biased into depletion, and an incident beam with a wavelength of 730 nm was partially transmitted from the readout side of the device through a leaky dielectric mirror (the mirror was tuned to the 450-650 nm spectral region). An image charge pattern was generated in the silicon and clocked in reverse, resulting in the generation of a sequential time-dependent signal at the output of the CCD's serial input register; this signal was converted into a video format to produce an image. This reverse mode operation did not produce an image directly from the CCD-LCLV, but rather only an electrical signal pattern that was converted into an image via additional video apparatus.
U.S. Pat. No. 4,227,201, "CCD Readout Structure for Display Applications", J. Grinberg et. al., issued Oct. 7, 1980 and assigned to Hughes Aircraft Company, the assignee of the present invention, discloses a liquid crystal light valve (LCLV) which uses the transfer of charge carriers representing a plurality of signals from a CCD array to a liquid crystal light modulated display medium. An interface structure for the storage and transfer of input data from a CCD array to a LCLV is described.
U.S. Pat. No. 4,319,239, "CCD Capacitance Modulation Matrix for Liquid Crystal Displays", C. P. Stephens, issued Mar. 9, 1982, and assigned to Hughes Aircraft Company, the assignee of the present invention, teaches controlling the optical response of a liquid crystal layer by an applied a.c. electric field having its amplitude in selected regions of the liquid crystal modulated by charge stored in an underlying charge transfer device. The necessity for an intrinsic substrate and the necessity for a d.c. electric field is eliminated because the charge packets stored by the CCD do not leave the CCD channel, but instead remain stationary while modulating the depletion capacitance beneath selected overlying regions of the liquid crystal display.
U.S. Pat. No. 4,198,647, "High Resolution Continuously Substituted Silicon Photodiode Substrate", J. Grinberg et. al., issued Apr. 15, 1980, assigned to Hughes Aircraft Company, the present assignee, discloses a semiconductor apparatus for the transfer of charge from one surface of a semiconductor substrate to the opposite surface through the use of a charge depletion region while maintaining their spatial resolution.
U.S. Pat. No. 4,191,452, "AC Silicon PN Junction Photodiode Light Valve Substrate", issued Mar. 4, 1980, assigned to Hughes Aircraft Company, the present assignee, discloses a single crystal silicon charge storage apparatus suitable for use in an alternating current driven LCLV having a PIN photodiode structure. The disclosed apparatus includes a structure which can be photoactivated or receive signal representing charge carriers from a CCD or any other source and convert these charge carriers into an AC signal that will activate the liquid crystal layer.
U.S. Pat. No. 4,169,231, "Buried Channel to Surface Channel CCD Charge Transfer Structure", J. G. Nash et. al., issued Sep. 25, 1979, also assigned to surface channel charge coupled device suitable for use in the present invention for high bandwidth imaging.
U.S. Pat. No. 4,443,064, "High Resolution AC Silicon MOS-Light Valve Substrate", J. Grinberg et. al., issued Apr. 17, 1984, assigned to Hughes Aircraft Company, the present assignee, discloses a CCD driven LCLV and an MOS capacitor type structure for the storage and transfer of photogenerated minority carrier representing signals to an alternating current driven LCLV.